1. Field of the Invention
The present invention relates to an R—Fe—B based rare-earth sintered magnet including crystal grains of an R2Fe14B type compound (where R is a rare-earth element) as a main phase and a method for producing such a magnet. More particularly, the present invention relates to an R—Fe—B based rare-earth sintered magnet, which includes a light rare-earth element RL (which is at least one of Nd and Pr) as a major rare-earth element R and in which a portion of the light rare-earth element RL is replaced with a heavy rare-earth element RH (which is at least one element selected from the group consisting of Dy, Ho and Tb).
2. Description of the Related Art
An R—Fe—B based rare-earth sintered magnet, including an Nd2Fe14B type compound phase as a main phase, is known as a permanent magnet with the highest performance, and has been used in various types of motors such as a voice coil motor (VCM) for a hard disk drive and a motor for a hybrid car and in numerous types of consumer electronic appliances. When used in motors and various other devices, the R—Fe—B based rare-earth sintered magnet should exhibit thermal resistance and coercivity that are high enough to withstand an operating environment at an elevated temperature.
As a means for increasing the coercivity of an R—Fe—B based rare-earth sintered magnet, a molten alloy, including a heavy rare-earth element RH as an additional element, may be used. According to this method, the light rare-earth element RL, which is included as a rare-earth element R in an R2Fe14B phase, is replaced with a heavy rare-earth element RH, and therefore, the magnetocrystalline anisotropy (which is a physical quantity that determines the coercivity) of the R2Fe14B phase improves. However, although the magnetic moment of the light rare-earth element RL in the R2Fe14B phase has the same direction as that of Fe, the magnetic moments of the heavy rare-earth element RH and Fe have mutually opposite directions. That is why the greater the percentage of the light rare-earth element RL replaced with the heavy rare-earth element RH, the lower the remanence Br would be.
Meanwhile, as the heavy rare-earth element RH is one of rare natural resources, its use is preferably cut down as much as possible. For these reasons, the method in which the light rare-earth element RL is entirely replaced with the heavy rare-earth element RH is not preferred.
To get the coercivity increased effectively with the addition of a relatively small amount of the heavy rare-earth element RH, it was proposed that an alloy or compound powder, including a lot of the heavy rare-earth element RH, be added to a main phase material alloy powder including a lot of the light rare-earth element RL and then the mixture be compacted and sintered. According to this method, the heavy rare-earth element RH is distributed a lot in the vicinity of the grain boundary of the R2Fe14B phase, and therefore, the magnetocrystalline anisotropy of the R2Fe14B phase can be improved efficiency on the outer periphery of the main phase. The R—Fe—B based rare-earth sintered magnet has a nucleation-type coercivity generating mechanism. That is why if a lot of the heavy rare-earth element RH is distributed on the outer periphery of the main phase (i.e., near the grain boundary thereof), the magnetocrystalline anisotropy of all crystal grains is improved, the nucleation of reverse magnetic domains can be minimized, and the coercivity increases as a result. At the core of the crystal grains that does not contribute to increasing the coercivity, no light rare-earth element RL is replaced with the heavy rare-earth element RH. Consequently, the decrease in remanence Br can be minimized there, too.
If this method is actually adopted, however, the heavy rare-earth element RH has an increased diffusion rate during the sintering process (which is carried out at a temperature of 1,000° C. to 1,200° C. on an industrial scale) and may diffuse to reach the core of the crystal grains, too. For that reason, it is not easy to obtain the expected crystal structure.
As another method for increasing the coercivity of an R—Fe—B based rare-earth sintered magnet, a metal, an alloy or a compound including a heavy rare-earth element RH is deposited on the surface of the sintered magnet and then thermally treated and diffused. Then, the coercivity could be recovered or increased without decreasing the remanence so much (see Patent Documents Nos. 1, 2 and 3).
Patent Document No. 1 teaches forming a thin-film alloy layer, including 1.0 at % to 50.0 at % of at least one element that is selected from the group consisting of Ti, W, Pt, Au, Cr, Ni, Cu, Co, Al, Ta and Ag and R′ as the balance (which is at least one element selected from the group consisting of Ce, La, Nd, Pr, Dy, Ho and Tb), on the surface of a sintered magnet body to be ground.
Patent Document No. 2 discloses that a metallic element R (which is at least one rare-earth element selected from the group consisting of Y, Nd, Dy, Pr, Ho and Tb) is diffused to a depth that is at least equal to the radius of crystal grains exposed on the uppermost surface of a small-sized magnet, thereby repairing the damage done on the machined surface and increasing (BH) max.
Patent Document No. 3 discloses that the magnetic properties could be recovered by depositing a CVD film consisting mostly of a rare-earth element on the surface of a magnet with a thickness of 2 mm or less.    Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 62-192566    Patent Document No. 2: Japanese Patent Application Laid-Open Publication No. 2004-304038    Patent Document No. 3: Japanese Patent Application Laid-Open Publication No. 2005-285859
All of the techniques disclosed in Patent Documents Nos. 1, 2 and 3 were developed to repair the damage done on the machined surface of a sintered magnet. That is why the metallic element, diffused inward from the surface, can reach no farther than a surface region of the sintered magnet. For that reason, if the magnet had a thickness of 3 mm or more, the coercivity could hardly be increased effectively.
Magnets for EPS and HEV motors, which are expected to expand their markets in the near future, need to be rare-earth sintered magnets with a thickness of at least 3 mm and preferably 5 mm or more. To increase the coercivity of a sintered magnet with such a thickness, a technique for diffusing the heavy rare-earth element RH efficiently throughout the inside of the R—Fe—B based rare-earth sintered magnet with a thickness of 3 mm or more needs to be developed.